Optoelectronic device with modulation doping

ABSTRACT

An improved heterostructure for an optoelectronic device is provided. The heterostructure includes an active region, an electron blocking layer, and a p-type contact layer. The p-type contact layer and electron blocking layer can be doped with a p-type dopant. The dopant concentration for the electron blocking layer can be at most ten percent the dopant concentration of the p-type contact layer. A method of designing such a heterostructure is also described.

REFERENCE TO RELATED APPLICATIONS

The current application claims the benefit of U.S. ProvisionalApplication No. 62/081,222, titled “Optoelectronic Device withModulation Doping,” which was filed on 18 Nov. 2014, and is acontinuation-in-part of U.S. application Ser. No. 14/475,638, titled“Optoelectronic Device with Modulation Doping,” which was filed on 3Sep. 2014, and which claims the benefit of U.S. Provisional ApplicationNo. 61/873,346, which was filed on 3 Sep. 2013, each of which is herebyincorporated by reference.

TECHNICAL FIELD

The disclosure relates generally to electronic and optoelectronicdevices, and more particularly, to group III nitride based electronicand optoelectronic devices.

BACKGROUND ART

Development of group III nitride based electronic and optoelectronicdevices with high efficiency and reliability depends on many factors,such as a quality of the semiconductor layers, active layer design, andcontact quality. In particular, designing highly conductive p-typegallium nitride (GaN) and/or aluminum gallium nitride (AlGaN) isimportant for a number of electronic and optoelectronic devices,including ultraviolet light emitting diodes (UV LEDs). Achieving a highp-type conductivity of magnesium (Mg)-doped AlGaN has been difficult dueto a large acceptor activation energy of 150-250 milli-electron Volts(meV), as well as due to a low hole mobility in heavily Mg-doped AlGaNalloys. The problem is particularly severe with increased molar fractionof aluminum due to a further increase of the acceptor activation energyand also due to an increase in unintentional donor concentration withthe increasing aluminum molar fraction. For AlGaN layers having highaluminum molar fractions, the oxygen (O) donor concentration can resultin insulating or even n-type characteristics of the AlGaN layers despiteheavy Mg doping.

Additionally, heavy Mg doping can negatively affect the reliability ofthe optoelectronic device. The existence of degradation beyond deviceself-heating has been previously observed and attributed to themigration of Al atoms from the p-type cladding.

One proposed degradation mechanism for group III nitride based LEDs iselectrons with high kinetic energies crossing the p-n junction, therebycausing a decrease in output power. This energy is transferred into thelattice, and more specifically to the electron blocking layer designedto confine electrons within the quantum wells of the active layer. Theenergy released by electrons assists in breaking both Mg-hydrogen (H)bonds, further activating carriers in the p-type layer, and Ga—N bonds,creating nitrogen vacancies, V_(N). The increased Mg activation causesan initial rise in output power before reaching a steady-state, whilethe V_(N) formation takes significantly longer to reach equilibrium andis responsible for the slow decrease in emission over a longer period oftime. Alternatively, released electron energy may contribute toformation of Mg—H₂ complexes and result in an overall decrease of p-typedoping. The energy of formation for the nitrogen vacancy in p-type AlGaNhas been calculated to be significantly lower than that of p-type GaN.However, the Mg—H₂ complex is more stable in AlGaN than in GaN. Thus, inhigh-Al content devices, almost all of the atom displacement leads toV_(N) formation, causing the slow further degradation observed in UVLEDs, which is manifested in an increase in the depletion edge on thep-side of the junction, which has been observed in thecapacitance-voltage data, and which further shows that this behavior isamplified at higher current densities and associated operatingtemperatures.

Formation of nitrogen vacancies and other defects due toelectron-lattice interaction results in effective trapping of holes insemiconductor layers. One approach to reduce degradation ofsemiconductor layers is through the use of a micro pixel device design,or by using LEDs with large planar area devices, which allow for reducedcurrent densities and operating temperatures, limiting the velocity ofelectrons approaching the p-n junction, electron blocking layer, andp-type layer.

SUMMARY OF THE INVENTION

The inventors propose a new solution for reducing the formation ofdefects and hole trapping in p-type doped semiconductor layers. In anembodiment of the solution described herein, such a reduction isobtained based on modulation doping. In this manner, a reliability ofthe corresponding semiconductor device can be improved. In anotherembodiment, the reliability is further improved through concurrentoptimization of several parameters of device heterostructure. Suchparameters can include, for example, a compositional profile and dopingprofile of one or more of the semiconductor layers. Furthermore,optimization of strains within one or more of the semiconductor layersand the resultant polarization fields can significantly influencereliability of the device.

Aspects of the invention provide an improved heterostructure for anoptoelectronic device. The heterostructure includes an active region, anelectron blocking layer, and a p-type contact layer. The p-type contactlayer and electron blocking layer can be doped with a p-type dopant. Thedopant concentration for the electron blocking layer can be at most tenpercent the dopant concentration of the p-type contact layer. A methodof designing such a heterostructure is also described.

A first aspect of the invention provides a heterostructure comprising:an active region; a p-type contact layer having a p-type contact layerdopant concentration; and an electron blocking layer located between theactive region and the p-type contact layer, wherein a p-type dopantconcentration in the electron blocking layer is at most ten percent ofthe p-type contact layer dopant concentration.

A second aspect of the invention provides an optoelectronic devicecomprising: an n-type contact layer having an n-type doping; a p-typecontact layer having a p-type contact layer dopant concentration; anactive region located between the n-type contact layer and the p-typecontact layer; and an electron blocking layer located between the activeregion and the p-type contact layer, wherein a p-type dopantconcentration in the electron blocking layer is at most ten percent ofthe p-type contact layer dopant concentration.

A third aspect of the invention provides a method of fabricating adevice, the method comprising: creating a device design for the deviceusing a computer system, wherein the device design includes aheterostructure comprising: an active region; a p-type contact layerhaving a target p-type contact layer dopant concentration; and anelectron blocking layer located between the active region and the p-typecontact layer, wherein a target p-type dopant concentration for theelectron blocking layer is at most ten percent of the target p-typecontact layer dopant concentration; and providing the device design foruse in fabricating the device according to the device design.

The illustrative aspects of the invention are designed to solve one ormore of the problems herein described and/or one or more other problemsnot discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various aspects of the invention.

FIG. 1 shows a schematic structure of an illustrative optoelectronicdevice according to an embodiment.

FIG. 2 shows a typical composition profile of a portion of aheterostructure for an optoelectronic device according to the prior art.

FIGS. 3A and 3B show illustrative composition profiles of a portion of agroup III nitride heterostructure for an optoelectronic device accordingto embodiments.

FIG. 4 shows illustrative composition profiles of the p-type side of anactive region with differing dopant concentrations according toembodiments.

FIG. 5A shows a composition profile of the p-type side of an activeregion according to the prior art and FIGS. 5B-5F show illustrativecomposition profiles of the p-type side of an active region withdiffering alloy compositions according to embodiments.

FIGS. 6A-6F show illustrative composition profiles of the p-type side ofan active region with differing electron blocking layer and/or p-typeinterlayer compositions according to embodiments.

FIGS. 7A and 7B show illustrative composition profiles of a p-typecontact layer according to embodiments.

FIGS. 8A-8C show a zero bias band diagram, a five volt bias banddiagram, and a carrier concentration diagram for an illustrativeheterostructure including an electron blocking layer according to anembodiment.

FIGS. 9A-9C show a zero bias band diagram, a five volt bias banddiagram, and a carrier concentration diagram for an illustrativeheterostructure including a graded electron blocking layer according toan embodiment.

FIGS. 10A and 10B show a five volt bias band diagram and a carrierconcentration diagram for an illustrative heterostructure including agraded electron blocking layer with two distinct graded regionsaccording to an embodiment.

FIGS. 11A-11D show band gap diagrams corresponding to illustrativesuperlattice configurations according to embodiments.

FIGS. 12A and 12B show illustrative composition and doping profiles of ap-type contact layer according to embodiments.

FIGS. 13A and 13B show illustrative composition doping and temperatureprofiles for forming a p-type contact layer according to embodiments.

FIG. 14 shows an illustrative flow diagram for fabricating a circuitaccording to an embodiment.

It is noted that the drawings may not be to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide an improvedheterostructure for an optoelectronic device. The heterostructureincludes an active region, an electron blocking layer, and a p-typecontact layer. The p-type contact layer and electron blocking layer canbe doped with a p-type dopant. The dopant concentration for the electronblocking layer can be at most ten percent the dopant concentration ofthe p-type contact layer. A method of designing such a heterostructureis also described.

As used herein, unless otherwise noted, the term “set” means one or more(i.e., at least one) and the phrase “any solution” means any now knownor later developed solution. As used herein, two materials havecomparable compositions when the molar fractions of the correspondingmaterials differ by at most ten percent (five percent in a more specificembodiment). For example, considering two group III nitride materials,Al_(x)In_(y)B_(z)Ga_(1-x-y-z)N and Al_(x′)In_(y′)B_(z′)Ga_(1-x′-y′-z′)N,the two materials have comparable compositions when each of the molarfractions x, y, and z differs from the corresponding molar fractions x′,y′, and z′ by less than ten percent, where the percentage is calculatedby taking a difference between the molar fractions and dividing thevalue by the higher molar fraction. Similarly, two layers havecomparable thicknesses when the corresponding thicknesses differ by atmost ten percent (five percent in a more specific embodiment). As alsoused herein, two dopant concentrations are comparable when they are onthe same order as one another. It is understood that two numbers are onthe same order as one another when a ratio of the higher number to thelower number is less than ten.

Aspects of the invention provide a heterostructure that can beincorporated into an optoelectronic device, such as a conventional orsuper luminescent light emitting diode (LED), a light emitting laser, alaser diode, a light sensor, a photodetector, a photodiode, an avalanchediode, and/or the like. Turning to the drawings, FIG. 1 shows aschematic structure of an illustrative optoelectronic device 10according to an embodiment. In a more particular embodiment, theoptoelectronic device 10 is configured to operate as an emitting device,such as a light emitting diode (LED). In this case, during operation ofthe optoelectronic device 10, application of a bias comparable to theband gap results in the emission of electromagnetic radiation from anactive region 18 of the optoelectronic device 10. The electromagneticradiation emitted by the optoelectronic device 10 can have a peakwavelength within any range of wavelengths, including visible light,ultraviolet radiation, deep ultraviolet radiation, infrared light,and/or the like. In an embodiment, the device 10 is configured to emitradiation having a dominant wavelength within the ultraviolet range ofwavelengths. In a more specific embodiment, the dominant wavelength iswithin a range of wavelengths between approximately 210 andapproximately 350 nanometers.

The optoelectronic device 10 includes a heterostructure comprising asubstrate 12, a buffer layer 14 adjacent to the substrate 12, an n-typelayer 16 (e.g., a cladding layer, electron supply layer, contact layer,and/or the like) adjacent to the buffer layer 14, and an active region18 having an n-type side 19A adjacent to the n-type layer 16.Furthermore, the heterostructure of the optoelectronic device 10includes a first p-type layer 20 (e.g., an electron blocking layer)adjacent to a p-type side 19B of the active region 18 and a secondp-type layer 22 (e.g., a cladding layer, hole supply layer, contactlayer, and/or the like) adjacent to the first p-type layer 20.

In a more particular illustrative embodiment, the optoelectronic device10 is a group III-V materials based device, in which some or all of thevarious layers are formed of elements selected from the group III-Vmaterials system. In a still more particular illustrative embodiment,the various layers of the optoelectronic device 10 are formed of groupIII nitride based materials. Group III nitride materials comprise one ormore group III elements (e.g., boron (B), aluminum (Al), gallium (Ga),and indium (In)) and nitrogen (N), such that B_(w)Al_(x)Ga_(y)In_(z)N,where 0≦W, X, Y, Z≦1, and W+X+Y+Z=1. Illustrative group III nitridematerials include binary, ternary and quaternary alloys such as, AlN,GaN, InN, BN, AlGaN, AlInN, AlBN, AlGaInN, AlGaBN, AlInBN, and AlGaInBNwith any molar fraction of group III elements.

An illustrative embodiment of a group III nitride based optoelectronicdevice 10 includes an active region 18 (e.g., a series of alternatingquantum wells and barriers) composed of In_(y)Al_(x)Ga_(1-x-y)N,Ga_(z)In_(y)Al_(x)B_(1-x-y-z)N, an Al_(x)Ga_(1-x)N semiconductor alloy,or the like. Similarly, both the n-type cladding layer 16 and the firstp-type layer 20 can be composed of an In_(y)Al_(x)Ga_(1-x-y)N alloy, aGa_(z)In_(y)Al_(x)B_(1-x-y-z)N alloy, or the like. The molar fractionsgiven by x, y, and z can vary between the various layers 16, 18, and 20.The substrate 12 can be sapphire, silicon carbide (SiC), silicon (Si),GaN, AlGaN, AlON, LiGaO₂, or another suitable material, and the bufferlayer 14 can be composed of AlN, an AlGaN/AlN superlattice, and/or thelike.

As shown with respect to the optoelectronic device 10, a p-type metal 24can be attached to the second p-type layer 22 and a p-type contact(electrode) 26 can be attached to the p-type metal 24. Similarly, ann-type metal 28 can be attached to the n-type layer 16 and an n-typecontact (electrode) 30 can be attached to the n-type metal 28. Thep-type metal 24 and the n-type metal 28 can form ohmic contacts to thecorresponding layers 22, 16, respectively. In an embodiment, the p-typemetal 24 and the n-type metal 28 each comprise several conductive andreflective metal layers, while the n-type contact 30 and the p-typecontact 26 each comprise highly conductive metal. In an embodiment, thesecond p-type layer 22 and/or the p-type contact 26 can be transparent(e.g., semi-transparent or transparent) to the electromagnetic radiationgenerated by the active region 18. For example, the second p-type layer22 and/or the p-type contact 26 can comprise a short period superlatticelattice structure, such as an at least partially transparent magnesium(Mg)-doped AlGaN/AlGaN short period superlattice structure (SPSL).Furthermore, the p-type contact 26 and/or the n-type contact 30 can bereflective of the electromagnetic radiation generated by the activeregion 18. In another embodiment, the n-type layer 16 and/or the n-typecontact 30 can be formed of a short period superlattice, such as anAlGaN SPSL, which is transparent to the electromagnetic radiationgenerated by the active region 18.

As used herein, a layer is a transparent layer when the layer allows atleast ten percent of radiation having a target wavelength, which isradiated at a normal incidence to an interface of the layer, to passthere through. Furthermore, as used herein, a layer is a reflectivelayer when the layer reflects at least ten percent of radiation having atarget wavelength, which is radiated at a normal incidence to aninterface of the layer. In an embodiment, the target wavelength of theradiation corresponds to a wavelength of radiation emitted or sensed(e.g., peak wavelength+/− five nanometers) by the active region 18during operation of the device 10. For a given layer, the wavelength canbe measured in a material of consideration and can depend on arefractive index of the material.

As further shown with respect to the optoelectronic device 10, thedevice 10 can be mounted to a submount 36 via the contacts 26, 30 in aflip chip configuration. In this case, the substrate 12 is located onthe top of the optoelectronic device 10. To this extent, the p-typecontact 26 and the n-type contact 30 can both be attached to a submount36 via contact pads 32, 34, respectively. The submount 36 can be formedof aluminum nitride (AlN), silicon carbide (SiC), and/or the like.

Any of the various layers of the optoelectronic device 10 can comprise asubstantially uniform composition or a graded composition. For example,a layer can comprise a graded composition at a heterointerface withanother layer. In an embodiment, the first p-type layer 20 comprises ap-type electron blocking layer having a graded composition. The gradedcomposition(s) can be included to, for example, reduce stress, improvecarrier injection, and/or the like. Similarly, a layer can comprise asuperlattice including a plurality of periods, which can be configuredto reduce stress, and/or the like. In this case, the composition and/orwidth of each period can vary periodically or a periodically from periodto period.

It is understood that the layer configuration of the optoelectronicdevice 10 described herein is only illustrative. To this extent, aheterostructure for an optoelectronic device can include an alternativelayer configuration, one or more additional layers, and/or the like. Asa result, while the various layers are shown immediately adjacent to oneanother (e.g., contacting one another), it is understood that one ormore intermediate layers can be present in a heterostructure for anoptoelectronic device. For example, an illustrative heterostructure foran optoelectronic device can include an undoped layer between the activeregion 18 and one or both of the second p-type layer 22 and the n-typelayer 16.

Furthermore, a heterostructure for an optoelectronic device can includea Distributive Bragg Reflector (DBR) structure, which can be configuredto reflect light of particular wavelength(s), such as those emitted bythe active region 18, thereby enhancing the output power of thedevice/heterostructure. For example, the DBR structure can be locatedbetween the second p-type layer 22 and the active region 18. Similarly,a heterostructure for an optoelectronic device can include a p-typelayer 20 located between the second p-type layer 22 and the activeregion 18. The DBR structure and/or the p-type layer 20 can have anycomposition selected based on a desired wavelength of the lightgenerated by the device. In one embodiment, the DBR structure has a Mg,Mn, Be, or Mg+Si-doped p-type composition. The p-type layer 20 can havea p-type AlGaN, AlInGaN, and/or the like. It is understood that aheterostructure for an optoelectronic device can include both the DBRstructure and the p-type layer 20 (which can be located between the DBRstructure and the second p-type layer 22) or can include only one of theDBR structure or the p-type layer 20. In an embodiment, the p-type layer20 can be included in the device/heterostructure in place of an electronblocking layer. In another embodiment, the p-type layer 20 can beincluded between the second p-type layer 22 and the electron blockinglayer.

Additional aspects of the invention are discussed in conjunction with agroup III nitride heterostructure. In particular, adjustment of aluminumcontent within layers of the group III nitride heterostructure is usedto illustrate aspects of the invention. However, it is understood thatthis is only illustrative, and the invention can be applied to varioustypes of materials and elements in the materials. Similarly, whileaspects of the invention are shown and described in conjunction with thep-type side of an active region, it is understood that embodiments ofthe invention can be applied to the design and configuration of one ormore layers located on the n-type side of the active region and/or theactive region itself.

FIG. 2 shows a typical composition profile of a portion of a group IIInitride heterostructure for an optoelectronic device according to theprior art. In this case, the heterostructure includes a n-type layer 2,an active region 4 including barriers 8A and quantum wells 8B, and anelectron blocking layer 6. The electron blocking layer 6 has a highaluminum content and is designed to trap electrons in the active region4. The electron blocking layer 6 can be followed by a graded interlayer7 and a p-type layer 8. The composition profile illustrates a percentageof aluminum present in each layer relative to the other group IIIelements as well as a doping profile for a dopant present in each layer.As illustrated, the percentage of aluminum present can be: approximatelyfifty percent in the n-type layer 2 and the barriers 8A; approximatelyforty percent for the quantum wells 8B; approximately sixty percent inthe electron blocking layer; between approximately twenty-five percentand approximately ten percent in the graded interlayer 7; andapproximately ten percent in the p-type layer 8. The composition profilealso illustrates the doping profiles 3, 5 for the various layers. Ingeneral, the n-type layer 2 and the active region 4 include an n-typedoping 3, such as silicon (Si) donors. The electron blocking layer 6,the graded interlayer 7, and the p-type layer 8 can each include ap-type doping 5. As illustrated by the corresponding doping profiles,each of the n-type doping 3 and p-type doping 5 is typically of aconstant doping concentration, e.g., approximately 1×10¹⁸ cm⁻³, betweenall of the layers.

In an embodiment, one or more of the p-type layers of theheterostructure of an optoelectronic device has a significantlydifferent doping profile (e.g., concentration of dopants) than that ofthe p-type doping 5 of the prior art. The doping profile(s) and/oradditional compositional fluctuations of the p-type layer(s) can beconfigured, for example, to improve internal quantum efficiency of thecorresponding device, e.g., through carrier localization, over thatprovided by devices of the prior art (e.g., having a composition profileas shown in FIG. 2). Furthermore, the doping profile(s) and/orcompositional fluctuations can improve a reliability of thecorresponding device over that of prior art devices, e.g., throughconfiguration of strains and resulting polarization fields within thesemiconductor layers.

FIGS. 3A and 3B show illustrative composition profiles of a portion of agroup III nitride heterostructure for an optoelectronic device, such asthe optoelectronic device 10 shown in FIG. 1, according to embodiments.As shown in FIG. 3A, the heterostructure includes an n-type layer 16(e.g., an n-type contact layer), an active region 18 (including aplurality of quantum wells and barriers), an electron blocking layer 20,and a p-type contact layer 22. Additionally, the heterostructure isshown including a p-type interlayer 21 located between the electronblocking layer 20 and the p-type contact layer 22. The electron blockinglayer 20 can have a thickness in a range of one to one hundrednanometers, while the p-type interlayer 21 can have a thickness in arange of ten to one thousand nanometers. In an embodiment, thecomposition profiles can correspond to a percentage of aluminum presentin each layer relative to the other group III elements as well as adoping profile for a dopant present in each layer. In a more particularembodiment, the percentages of aluminum are similar to those describedin conjunction with FIG. 2. However, it is understood that this is onlyillustrative and each layer can have any type of material selected basedon one or more target attributes of the layer and/or correspondingdevice.

In general, the electron blocking layer 20 is configured to trapelectrons within the active region 18. To do so, a band gap of theelectron blocking layer 20 can be higher than a typical band gap of thebarriers 17A located within the active region 18. One approach forincreasing the band gap of a group III-V semiconductor layer is toincrease the aluminum content in the layer. In an embodiment, at least aportion (e.g., three atomic monolayers or more) of the electron blockinglayer 20 has an aluminum molar fraction at least three percent higherthan the aluminum molar fraction of the barrier 17A in the active region18 with the highest aluminum molar fraction. When the active region 18includes barriers 17A having different and/or varying aluminum molarfractions, the aluminum molar fraction of the portion of the electronblocking layer 20 can exceed the highest aluminum molar fraction locatedwithin the active region 18 by at least three percent.

The n-type layer 16 and the active region 18 can include an n-typedoping 40 using any appropriate donors, such as silicon (Si) donors, orthe like. As illustrated, the n-type doping 40 can provide a constantdoping concentration across the layers 16, 18, e.g., approximately1×10¹⁸ cm⁻³, which produces a uniform n-type doping profile 40. However,as shown in FIGS. 3A and 3B, the p-type layers 20, 21, 22 can have anon-uniform p-type doping profile 42A, 42B using any appropriateacceptors, such as magnesium (Mg) acceptors, or the like. In particular,a doping concentration for the p-type doping in the electron blockinglayer 20 is substantially less than that of the doping concentration ofthe p-type doping in the contact layer 22. The p-type doping 42A in thecontact layer 22 can have a dopant concentration density in a range of1×10¹⁸ cm⁻³ and 1×10¹⁹ cm⁻³. In an embodiment, the doping concentrationof the electron blocking layer 20 is at most ten percent of the dopingconcentration of the p-type contact layer 22. Conversely, an aluminumcontent (i.e., the aluminum molar fraction indicated by the x-valueshown in FIG. 3B) of the electron blocking layer 20 can be significantlyhigher than the aluminum content of the p-type contact layer 22.

When included, the p-type interlayer 21 can provide a transition betweenthe electron blocking layer 20 and the p-type contact layer 22. However,it is understood that the physical properties of the p-type interlayer21 are different from the physical properties of both the electronblocking layer 20 and the p-type contact layer 22. The p-type interlayer21 can be configured to act as a stress relieving layer, e.g., torelieve stress in the electron blocking layer 20 and the p-type contactlayer 22, which would otherwise be present due to a large latticemismatch between the electron blocking layer 20 and the p-type contactlayer 22. Furthermore, the p-type interlayer 21 can be configured toreduce stress present in the active region 18. A reduction of the stressis accomplished by reducing the lattice mismatch between the electronblocking layer 20 and p-type contact layer 22 through the inclusion ofthe p-type interlayer 21. The p-type interlayer 21 creates a gradualtransition from a layer with relatively small lattice constant (e.g.,the electron blocking layer 20) to a layer with a relatively largelattice constant (e.g., the p-type contact layer 22).

To reduce the stress, any combination of one or more solutions can beutilized. For example, the p-type interlayer 21 can be configured tohave an effective lattice constant that is between the lattice constantsof the electron blocking layer 20 and the p-type contact layer 22. In anembodiment, the p-type interlayer 21 has a variable composition profile,such as a variable doping concentration, a variable aluminum content,and/or the like. The variable composition can be configured to provide atransition from a composition comparable to (e.g., the same as) theelectron blocking layer 20 on a side of the p-type interlayer 21adjacent to the electron blocking layer 20 to a composition comparableto the p-type contact layer 22 on a side of the p-type interlayer 21adjacent to the p-type contact layer 22. In this manner, the effectivelattice constant of the p-type interlayer 21 can change from a latticeconstant comparable to that of the electron blocking layer 20 to alattice constant comparable to that of the p-type contact layer 22,thereby resulting in an overall reduction of stress over that presentwhen the p-type interlayer 21 is not included.

Furthermore, the p-type interlayer 21 can have a variable p-type dopantconcentration. To this extent, as illustrated by the p-type dopingprofiles 42A, 42B, the p-type interlayer 21 can have a p-type dopantconcentration that is comparable to the dopant concentration in theelectron blocking layer 20 on the side of the p-type interlayer 21adjacent to the electron blocking layer 20 and a p-type dopantconcentration that is comparable to the p-type dopant concentration inthe p-type contact layer 22 on the side of the p-type interlayer 21adjacent to the p-type contact layer 22. The dopant concentration withinthe p-type interlayer 21 can transition from a lower dopantconcentration to a higher dopant concentration using any solution. Forexample, the dopant concentration can continuously increase, increase atvarying rates, increase in a stepwise manner, and/or the like.Furthermore, the transition can occur across all of a height of thep-type interlayer 21 and/or over only a portion of the height of thep-type interlayer 21.

In an embodiment, the p-type interlayer 21 can have a variable alloycomposition. To this extent, the p-type interlayer 21 can have, forexample, an aluminum content that varies from an aluminum content thatis comparable to the aluminum content in the electron blocking layer 20on the side of the p-type interlayer 21 adjacent to the electronblocking layer 20 and an aluminum content that is comparable to thealuminum content in the p-type contact layer 22 on the side of thep-type interlayer 21 adjacent to the p-type contact layer 22. Thealuminum content within the p-type interlayer 21 can transition from ahigher aluminum content to a lower aluminum content using any solution.For example, the aluminum content can continuously decrease, decrease atvarying rates, decrease in a stepwise manner, and/or the like.Furthermore, the transition can occur across all of a height of thep-type interlayer 21 and/or over only a portion of the height of thep-type interlayer 21. In a more particular embodiment, the p-typeinterlayer 21 can comprise a group III nitride layer formed ofGa_(z)In_(y)Al_(x)B_(1-x-y-z)N, where 0≦x, y, z≦1, and 0≦x+y+z≦1.Furthermore, the molar fractions x, y, z can change by at least fivepercent throughout the entire thickness of the p-type interlayer 21. Thechange in molar fraction(s) can be gradual and continuous correspondingto a graded layer. In an embodiment, the change can comprise a series ofsteps. For example, the electron blocking layer 20 can comprise a molarfraction of aluminum of 0.7 and the p-type interlayer 21 can be gradedfrom the molar fraction of 0.7 to a molar fraction of 0.2 along itsthickness (an overall change in the aluminum molar fraction of about71%).

In the illustrative embodiments shown in FIGS. 3A and 3B, the p-typeinterlayer 21 includes a first portion adjacent to the electron blockinglayer 20 in which the alloy composition transitions from a compositioncomparable to that of the electron blocking layer 20 to a compositioncomparable to that of the composition of the p-type contact layer 22while the dopant concentration remains comparable to the dopantconcentration in the electron blocking layer 20. Once the alloycomposition in the interlayer 21 is comparable to that of the p-typecontact layer 22, the interlayer 21 can include a second portionadjacent to the p-type contact layer 22 in which the dopantconcentration transitions from a dopant concentration comparable to thatof the electron blocking layer 20 to a dopant concentration comparableto that of the p-type contact layer 22. In this manner, the dopantconcentration is not increased until the alloy composition in the p-typeinterlayer 21 is comparable to that of the p-type contact layer 22(e.g., a low aluminum content). However, it is understood that this isonly an illustrative configuration for the interlayer 21 and variousother configurations are possible.

For example, FIG. 4 shows illustrative composition profiles, each with aunique p-type doping profile 42C-42F, of the p-type side of an activeregion 18 according to embodiments. As illustrated, the p-typeinterlayer 21, as well as the electron blocking layer 20 and the p-typecontact layer 22, can have a substantially constant, comparable dopantconcentration, as shown by doping profile 42C. Alternatively, as shownby doping profiles 42D-42F, the electron blocking layer 20 can have adopant concentration that is lower than the dopant concentration of thep-type contact layer 22 (e.g., at least ten times lower), and the p-typeinterlayer 21 can have a varying dopant concentration that transitionsfrom the lower dopant concentration to the higher dopant concentration.For each doping profile 42D-42F, the dopant concentration varieslinearly and continuously in the portion of the p-type interlayer 21adjacent to the p-type contact layer 22. Additionally, in each case, adopant concentration comparable to the target dopant concentrationwithin the p-type contact layer 22 is achieved in the interlayer 21. Aspecific doping profile can be optimized based on the thickness and thecomposition (e.g., the aluminum molar fraction) of the electron blockinglayer 20, which in turn can be optimized for a particular configurationof the active region 18 using any solution.

Still further, an embodiment of the interlayer 21 can include aplurality of alternating tensile and compressive sublayers. Such aconfiguration can be fabricated, for example, by changes in an epitaxialgrowth procedure used to grow the interlayer 21. In an embodiment, theinterlayer 21 is grown as shown and described in U.S. patent applicationSer. No. 13/756,806, titled “Epitaxy Technique for Reducing ThreadingDislocations in Stressed Semiconductor Compounds,” which was filled on 1Feb. 2013, and which is hereby incorporated by reference. In this case,the compressive and tensile sublayers can be grown using a set ofepitaxial growth periods. During each epitaxial growth period, a firstsemiconductor sublayer having one of: a tensile stress or a compressivestress is grown followed by growth of a second semiconductor sublayerhaving the other of: the tensile stress or the compressive stressdirectly on the first semiconductor sublayer. One or more of a set ofgrowth conditions, a thickness of one or both of the sublayers, and/or alattice mismatch between the sublayers can be configured to create atarget level of compressive and/or shear stress within a minimumpercentage of the interface between the sublayers. The target level ofcompressive and/or shear stress can be selected based on an amount ofshear stress sufficient to turn a minimum set of dislocations in theinterlayer 21 from a direction primarily in a direction of a c-axis to adirection primarily in a c-plane of the interlayer 21. The growthconditions adjusted to affect the tensile and compressivecharacteristics of layers can include a molar ratio of group Vprecursors to group III precursors (V/III ratio), growth temperature,gas flow, and/or the like. As used herein, a “tensile layer” is a layerexperiencing tensile stress, and a “compressive layer” is a layerexperiencing compressive stress.

Various alternative alloy compositions also can be implemented accordingto embodiments. To this extent, FIG. 5A shows a composition profile 50Aof the p-type side of an active region according to the prior art andFIGS. 5B-5F show illustrative composition profiles 50B-50F of the p-typeside of an active region with differing alloy compositions according toembodiments. In each case, the composition profile 50A-50F is shown inconjunction with a percent of aluminum present in the correspondinggroup III-V material. In FIG. 5A, a quantum well (lower portion) betweenadjacent barriers is shown according to a prior art active region 4. Onthe p-type side of the active region 4, the heterostructure includes anelectron blocking layer 6, a graded interlayer 7, and a p-type contactlayer 8.

Embodiments described herein can alter one or more aspects of the p-typeside composition profile of the heterostructure. For example,embodiments can include a graded transition between a composition in anactive region 18 and a composition of an electron blocking layer 20. Thegraded transition can be configured to mediate stresses associated witha lattice mismatch between the electron blocking layer 20 and the lastquantum well formed on the p-type side of the active region 18.Mediation of such stresses can increase a reliability of theheterostructure and resulting device. For example, during operation of adevice, built in stresses can be relaxed through a dislocationgeneration process. Such dislocations lead to carrier capture andoverall deterioration in the operation of the device. Insertion of agraded transition can reduce a number of dislocations formed throughsuch a process.

Any of various graded transition solutions can be utilized. For example,the composition profile 50B illustrates an embodiment in which thegraded transition is increased linearly, and starts from the compositionof the last quantum well on the p-type side of the active region 18going to the composition of the electron blocking layer 20. Similarly,the composition profile 50C illustrates an embodiment in which thegraded transition is increased in a nonlinear manner from thecomposition of the last quantum well on the p-type side of the activeregion 18 to a peak level in the electron blocking layer 20. In thecomposition profile 50E, the graded transition begins after the lastquantum well, but at a level corresponding to a barrier in the activeregion 18. In the composition profile 50F, the graded transition startsafter the last barrier in the active region 18. It is understood thatthese embodiments are only illustrative of various solutions for forminga graded transition between the active region 18 and the electronblocking layer 20. In an illustrative embodiment, a grading rate caninclude a change of 0.1 in a molar fraction of aluminum over a thicknessof few tens of nanometers (e.g., approximately thirty nanometers). Thegrading can be employed, for example, when an aluminum molar fraction ofthe electron blocking layer 20 is at least five percent higher than thecorresponding molar fraction of the p-type contact layer 22.

Similarly, a heterostructure can include a graded transition between theelectron blocking layer 20 and the p-type contact 22, which can reduce alattice mismatch between the layers. For example, the compositionprofile 50B includes an interlayer 21 having a linearly gradedtransition, which starts at a composition approximately equal to acomposition in a quantum well in the active region 18 and ends at thecomposition of the p-type contact 22. In the composition profile 50C, anentirety of the electron blocking layer 20 has a substantiallycontinuously changing composition, which provides a reduced effectivelattice constant for the electron blocking layer 20. As illustrated, thechanging composition can be configured to provide a graded transitionfrom a peak composition in the electron blocking layer 20 to thecomposition of the p-type contact 22. In each of the compositionprofiles 50D-50F, an interlayer 21 includes a first portion that have alinearly graded transition from that of the electron blocking layer 20to an intermediate level, at which the composition remains in a secondportion of the interlayer 21 prior to the p-type contact layer 22. Eachof the composition profiles 50D-50F is shown including an intermediatecomposition at which the graded portion stops and which is differentfrom the composition of the p-type contact layer 22. A sharp differencebetween the composition of the interlayer 21 and the p-type contactlayer 22 may be included, for example, to yield generation ofdislocations at the interface of the interlayer 21 and the p-typecontact layer 22, which can result in relaxation of the p-type contactlayer 22.

It is understood that for each of the illustrative composition profiles50B-50F, a concentration of a p-type dopant in the electron blockinglayer 20 can be significantly lower (e.g., at most ten percent) than thep-type dopant concentration in the p-type contact layer 22.

Various other solutions can be utilized to reduce the overall stressesassociated with including an electron blocking layer 20 while preservinga good quality of the semiconductor heterostructure. For example, FIGS.6A-6F show illustrative composition profiles 52A-52F of the p-type sideof an active region 18 with differing electron blocking layer 20 and/orp-type interlayer 21 compositions according to embodiments. In eachcomposition profile 52A-52F, an alloy composition of the electronblocking layer 20 and/or the p-type interlayer 21 is periodically or aperiodically adjusted in a manner to reduce the stresses present in thecorresponding heterostructure.

The composition profile 52A illustrates an electron blocking layer 20having a plurality of sublayers, each of which is defined by an alloycomposition (e.g., aluminum molar fraction) that differs from theimmediately adjacent sublayer(s). In an embodiment, the difference inalloy compositions, as expressed as the corresponding aluminum molarfractions, of two immediately adjacent sublayers is at least 0.5%. Amaximum variation in composition can be determined based on thethicknesses of the sublayers. For example, for sublayers havingthicknesses of a few nanometers, the sublayers can comprisesubstantially different compositions. However, thicker sublayers arelimited to less abrupt changes in composition. In an embodiment, amaximum variation in composition and sublayer thickness are selectedsuch as to not introduce additional dislocations within the electronblocking layer 20, e.g., by limiting a resulting stress times thicknessof 5 [GPA×nm].

As shown in the composition profile 52A, the sublayers can form astaircase compositional grading, which increases from the alloycomposition of the last quantum well in the active region 18 to a peakalloy composition (e.g., an aluminum molar fraction of approximately0.7) located in a central portion of the electron blocking layer 20before forming a decreasing staircase compositional grading to an alloycomposition of the p-type contact layer 22. In an embodiment, athickness of each of the sublayers is in a range between approximately0.5 nanometers and approximately 50 nanometers. In contrast tocontinuous compositional grading, use of a stepped (staircase)compositional grading can result in the formation of higher qualitysemiconductor layers for particular epitaxial growth methodologies.During epitaxial growth, a slow variation of growth parameters can beutilized to achieve high quality semiconductor layers, which istypically more difficult with continuous compositional grading which canresult in non-uniformities within the layer.

The composition profiles 52B, 52C illustrate sublayers of an electronblocking layer 20 forming a superlattice, in which the sublayersalternate between a relatively high aluminum content and a relativelylow aluminum content as compared to the immediately adjacent sublayer(s)in the electron blocking layer 20. In an embodiment, the aluminum molarfraction for sublayers having a relatively low aluminum content can bein a range between 0 to 0.6, while the aluminum molar fraction forsublayers having a relatively high aluminum content can be in a rangebetween 0.1 and 1. Regardless, a difference between the correspondingaluminum molar fractions of two immediately adjacent sublayers can be atleast 0.5% with the difference in compositions being selected such thatdislocations are not formed within the sublayers. In the compositionprofile 52B, the sublayers alternate between a high aluminum molarfraction and a low aluminum molar fraction, both of which remainsubstantially constant across a height of the electron blocking layer20.

Variation in the high and/or low aluminum molar fractions can alterstresses present within the electron blocking layer 20. In thecomposition profile 52C, the high aluminum molar fractions graduallyincrease from lower molar fractions to the highest molar fractions in acentral portion of the electron blocking layer 20 before graduallydecreasing to lower molar fractions closer to the p-type contact layer22. The low aluminum molar fractions remain substantially constantacross the electron blocking layer 20 until going lower close to thep-type contact layer 22. However, it is understood that the low aluminummolar fractions can vary in a manner similar to that shown for the highaluminum molar fractions. In an embodiment, a relative differencebetween the aluminum molar fractions of similar neighboring high/lowaluminum molar fraction sublayers is at least 0.5%.

In addition, the thicknesses of the sublayers in an electron blockinglayer 20 can change throughout the layer 20. Altering the thicknesses ofsublayers can provide additional control over an effective latticeconstant of the electron blocking layer 20, which can result inadditional control over the stresses in the layer 20. For example, asillustrated in the composition profiles 52B, 52C, a thickness of thesublayers having the high aluminum molar fractions gradually increasefrom the outer portions of the electron blocking layer 20 to the centralportion of the electron blocking layer 20. In contrast, the thickness ofthe sublayers having the low aluminum molar fractions can remainsubstantially constant. In an embodiment, a thickness of a sublayerdiffers from a thickness of at least one other sublayer by at least 1%with the difference in compositions being selected such thatdislocations are not formed within the sublayers. In another embodiment,neighboring high aluminum content sublayers differ in thickness by atleast 1%. In yet another embodiment, the neighboring sublayers with lowaluminum content differ in thickness by at least 1%. In still anotherembodiment, the thicknesses of the high aluminum content sublayersincrease towards a central portion of the electron blocking layer 20,while the thicknesses of the low aluminum content sublayers decreasestowards the central portion of the electron blocking layer 20, with thedifference in thicknesses of neighboring sublayers with similar aluminumcontent is at least 1%. It is understood that these configurations ofsublayer thicknesses are only illustrative, and an embodiment of anelectron blocking layer 20 can have any of various combinations ofsublayer thicknesses.

It is understood that the illustrative aluminum molar fractionvariations shown in the composition profiles 52A-52C can be combinedwith any of various other features and/or implemented in another layer,such as the interlayer 21. For example, the composition profile 52Dshows an interlayer 21 having a plurality of sublayers, each of which isdefined by an alloy composition (e.g., aluminum molar fraction) thatdiffers from the immediately adjacent sublayer(s). In an embodiment, thedifference in alloy compositions, as expressed as the correspondingaluminum molar fractions, of two immediately adjacent sublayers is atleast 0.5%. As shown in the composition profile 52D, the sublayers canform a staircase compositional grading, which decreases from the alloycomposition of the electron blocking layer 20 to an alloy composition ofthe p-type contact layer 22. In an embodiment, a thickness of each ofthe sublayers is in a range between approximately 0.5 nanometers andapproximately 50 nanometers.

Similarly, the composition profile 52D shows a staircase doping profile54D for the doping concentration in the interlayer 21, which increasesin steps from the doping concentration of the electron blocking layer 20to a doping concentration of the p-type contact layer 22. In anembodiment, the steps of the compositional grading and dopingconcentration are located in the same location within the interlayer 21.However, it is understood that this is only illustrative, and the stepscan be misaligned or only some of the steps can be aligned. Similarly,while the compositional grading and doping concentration are shownincluding the same number of steps, it is understood that a differentnumber of steps can be utilized for each.

The composition profile 52E illustrates a superlattice alloy compositionfor the electron blocking layer 20 combined with a graded compositionfor the interlayer 21. It is understood that the superlattice alloycomposition for the electron blocking layer 20 is only illustrative andan alternative superlattice configuration, such as those shown incomposition profiles 52B, 52C, can be used in conjunction with thegraded interlayer 21. The composition profile 52F illustrates aninterlayer 21 having a superlattice-type decreasing alloy composition.As illustrated, a thickness of the high aluminum content sublayers candecrease with distance from the electron blocking layer 20.

FIGS. 6A-6F also show an illustrative inverse dependence of the dopingprofile and the alloy composition (e.g., aluminum molar fraction). In anembodiment, the proportionality constant between the doping and thealuminum molar fraction can be selected so that the doping concentrationis in the range of approximately 10¹⁸ to approximately 10²⁰ dopants percubic centimeter when the aluminum molar fraction is zero. In anotherembodiment, when the aluminum molar fraction is between approximately0.5 to approximately 0.7, the doping concentration can be at most 10% ofthe doping concentration at zero aluminum molar fraction. In anembodiment, a dependence between the doping concentrations on theinverse value of the aluminum molar fraction is linear.

Variation in aluminum molar fractions and/or doping concentration alsocan be implemented in the p-type contact layer 22. To this extent, FIGS.7A and 7B show illustrative composition profiles 56A, 56B of a p-typecontact layer 22 according to embodiments. In each composition profile56A, 56B, the p-type contact layer 22 is shown including a plurality ofsublayers with alternating high and low aluminum molar fractions,thereby forming a superlattice adjacent to the interlayer 21. Inclusionof a superlattice can improve transparent and/or reflective propertiesof the p-type contact layer 22. Additionally, in the composition profile56B, the p-type contact layer 22 is shown having a modulated dopingprofile 58B that alternates between high and low doping concentrationsin the sublayers. The modulated doping profile 58B can improve holemobility and/or promote ionization of acceptors. As illustrated, thedoping concentrations and aluminum molar fractions can be adjusted atsubstantially the same time such that the doping concentrations andaluminum molar fractions are both concurrently high or low. In thiscase, the high aluminum molar fraction sublayers of the superlattice(e.g., the barriers) can be highly p-type doped, while the low aluminummolar fraction sublayers of the superlattice (e.g., the quantum wells)can be lightly p-type doped. However, it is understood that this is onlyillustrative of various possible embodiments. In an illustrativeembodiment, the superlattice in a composition profile 56A, 56B is anAl_(x)Ga_(1-x)N/Al_(y)Ga_(1-y)N superlattice. In a more particularillustrative embodiment, a width of the sublayers can be few nanometers,and a difference in molar fraction of the sublayers can be as much as 1,but preferably less than 0.5.

In an embodiment, one or more of the electron blocking layer 20, p-typeinterlayer 21, or p-type contact layer 22 has a laterally inhomogeneousalloy composition and/or doping concentration, which is configured toimprove conductivity while maintaining a relatively low absorption ofelectromagnetic radiation. For example, an embodiment of a layer 20, 21,22 can be configured as shown and described in U.S. patent applicationSer. No. 13/572,446, titled “Emitting Device with Compositional andDoping Inhomogeneities in Semiconductor Layers,” which was filed on 10Aug. 2012, and which is hereby incorporated by reference. In this case,one or more layers can include lateral regions configured to facilitatethe transmission of radiation through the layer and lateral regionsconfigured to facilitate current flow through the layer. The layer cancomprise a short period superlattice, which includes barriersalternating with wells. In this case, the barriers can include bothtransparent regions, which are configured to reduce an amount ofradiation that is absorbed in the layer, and higher conductive regions,which are configured to keep the voltage drop across the layer within adesired range.

For example, the p-type interlayer 21 and/or the p-type contact layer 22can have a non-uniform lateral dopant concentration and/or non-uniformlateral composition. In one embodiment, dopant concentration is at least5% different between highs and lows of the dopant concentrationlandscape, wherein the dopant concentration landscape is referred todopant concentration as a function of lateral (i.e., the plane of thelayer that is substantially parallel with the surface of the layeradjacent to another layer of the heterostructure) x, y coordinates ofthe corresponding layer 21, 22. Similarly, in an embodiment, theelectron blocking layer 20, the p-type interlayer 21, or the p-typecontact layer 22 may have non-uniform composition in the lateraldirection. For example, the change in aluminum molar fraction of thelayer 20, 21, 22 can be at least kT (measured at room temperature,˜0.026 eV) between highs and lows of the aluminum molar fractionlandscape, wherein the aluminum molar fraction landscape is referred toaluminum molar fraction as a function of lateral x, y coordinates of thecorresponding layer 20, 21, 22.

The inventors propose a scaling solution in order to determine a targetdoping concentration that will maintain a minimum reliability for a newoptoelectronic device being designed. In particular, using knownattributes of a model optoelectronic device having an acceptablereliability and a similar heterostructure (e.g., a heterostructureincluding one or more corresponding layers as the new optoelectronicdevice formed using comparable materials) with different sizedimensions, the scaling solution can be used to determine a dopingconcentration that will provide a similar (e.g., the same) level ofreliability for the new optoelectronic device having differentdimensions, e.g., a different lateral area and/or thickness of thep-type contact layer 22.

For example, assume a p-type contact layer of the model optoelectronicdevice has a doping concentration, N₀, a lateral p-type contact area,A₀, and a p-type contact layer thickness, d₀. An average number ofcollisions that an electron makes in traversing a scatterer, v, is givenby:v=σN ₀ d ₀,where σ is a scattering cross section of the acceptor (e.g., magnesium).For a flux of electrons with a current density, J₀, an average number ofcollisions per unit area, f₀, is given by:f ₀ =J ₀ σN ₀ d ₀.

Consider a new optoelectronic device having a lateral p-type contactarea, A, a p-type contact layer thickness, d, and a requirement that thetotal current, I, remains the same as in the model optoelectronicdevice. In this case, the following equation applies:I=J×A=J ₀ ×A ₀.The average number of collisions per unit area, f, for the newoptoelectronic device is given by:f=JσNd,and f≦f₀, to maintain the reliability of the model optoelectronic devicein the new optoelectronic device. In this case, we can determine that:

$\left. {{JNd} \leq {J_{0}N_{0}d_{0}}}\rightarrow\left. {{\frac{I}{A}{Nd}} \leq {\frac{I}{A_{0}}N_{0}d_{0}}}\rightarrow{N \leq {{N_{0}\left( \frac{A}{A_{0}} \right)}{\left( \frac{d_{0}}{d} \right).}}} \right. \right.$

The expression above provides a scaling algorithm of a target dopingconcentration in order to meet reliability constraints of the newoptoelectronic device. In particular, the expression indicates that thedopant concentration of the new optoelectronic device should scale asthe area ratio of the new p-type contact layer to the model p-typecontact layer, and scale inversely to the thickness ratio of the newp-type contact layer to the model p-type contact layer.

An electron blocking layer 20 can act as a significant barrier to holes,thereby reducing a concentration of holes in the active region 18. Forexample, FIGS. 8A-8C show a zero bias band diagram, a five volt biasband diagram, and a carrier concentration diagram for an illustrativeheterostructure including an electron blocking layer 20 according to anembodiment. In particular, as shown in the band diagrams of FIGS. 8A and8B, the heterostructure includes: an active region 18 with five quantumwells 17B separated by barriers 17A; an electron blocking layer 20; aninterlayer 21; and a p-type contact layer 22. The electron blockinglayer 20 can have an aluminum molar fraction in the range of 0.5-0.8,and can be 5-100 nm thick. In one embodiment the electron blocking layer20 has an aluminum molar fraction x=0.7 and is several tens ofnanometers thick. As illustrated by FIG. 8B, when under a bias of fivevolts, the electron blocking layer 20 provides a significant barrier toholes entering the active region 18 as shown by an abrupt change in bandgap between the electron blocking layer 20 and the interlayer 21.

In an embodiment, the barrier for the holes provided by the electronblocking layer 20 is reduced by grading the electron blocking layer 20.To this extent, FIGS. 9A-9C show a zero bias band diagram, a five voltbias band diagram, and a carrier concentration diagram for anillustrative heterostructure including a graded electron blocking layer20 according to an embodiment. As illustrated by FIG. 9B, the gradedelectron blocking layer 20 does not produce a significant barrier forholes. To this extent, by comparing the carrier concentration diagramsof FIGS. 8C and 9C, the graded electron blocking layer 20 increases theconcentration of holes in the quantum well adjacent to electron blockinglayer 20 by an order of magnitude.

However, the graded electron blocking layer 20 can reduce electroninjection current due to a reduced barrier for electrons. In anembodiment, the graded electron blocking layer 20 includes two distinctgraded regions. For example, FIGS. 10A and 10B show a five volt biasband diagram and a carrier concentration diagram for an illustrativeheterostructure including a graded electron blocking layer with twodistinct graded regions 20A, 20B according to an embodiment. The firstgraded region 20A comprises a relatively slow grading with a totalcompositional change in the molar fraction of aluminum on the order of0.3 for a graded region 20A having a thickness on the order of onehundred nanometers. The second graded region 20B can have asignificantly higher grading, such as a total compositional change inthe molar fraction of aluminum on the order of 0.3 for a graded region20A having a thickness on the order of five to fifty nanometers. It isunderstood that a portion of the increased hole concentration shown inFIG. 10B is caused by polarization doping resulting from the grading.Additionally, a large concentration of holes (e.g., approximately 1×10¹⁹cm⁻³) is present in the graded region 20B, where the grading issignificant.

It is understood that the grading described herein in conjunction withthe p-type layers 20, 21, 22 can be combined with forming a superlatticein a region of the layer. For example, the superlattice can havebarriers which are graded throughout the superlattice region, e.g., eachsuccessive barrier within a superlattice region is either smaller orhigher in aluminum molar fraction that the preceding barrier, thesesuperlattice configurations can be referred to as a down-gradedsuperlattice based on barriers or an up-graded superlattice based onbarriers, respectively. In an embodiment, the successive barrier has analuminum molar fraction that is at least 1% lower or higher than themolar fraction of the previous barrier. Additionally, it is understoodthat a quantum well height can be made progressively smaller or higherin a superlattice.

FIGS. 11A-11D show band gap diagrams corresponding to each of theillustrative superlattice configurations according to embodiments. Inparticular, FIG. 11A shows a down-graded superlattice based on barriers;FIG. 11B shows an up-graded superlattice based on barriers; FIG. 11Cshows a down-graded superlattice based on quantum wells; and FIG. 11Dshows an up-graded superlattice based on quantum wells. Still further,while variations in barrier or quantum well heights are shown, it isunderstood that configurations including varying barrier and quantumwell heights and/or varying barrier and/or quantum well thicknesses canbe utilized.

While the interlayer 21 is primarily described as having a differentp-type doping and/or different semiconductor alloy composition from bothof the electron blocking layer 20 and the p-type contact layer 22, it isunderstood that other solutions can be utilized to provide theinterlayer 21 with different physical properties than those of theelectron blocking layer 20 and the p-type contact layer 22. For example,the p-type interlayer 21 can be grown using a different growthmorphology than those used in growing the electron blocking layer 20 andthe p-type contact layer 22. For example, the growth morphology for theinterlayer 21 can differ by one or more of: growth conditions (e.g.,V/III ratio, temperature, and/or the like); an effective latticeconstant; a presence of defects; and/or the like.

FIGS. 12A and 12B show illustrative composition and doping profiles of ap-type contact layer 22 according to embodiments. The p-type contactlayer 22 can include a plurality of sublayers with alternating high andlow aluminum molar fractions to form a superlattice, similar to thep-type contact layer 22 shown in FIGS. 7A and 7B. However, in theembodiment shown in FIG. 12A, the electron blocking layer 20 can includea plurality of very thin sublayers 60. In the embodiment shown in FIG.12B, both the electron blocking layer 20 and the p-type interlayer 21can include the plurality of thin sublayers 60. However, it isunderstood an embodiment also can comprise only the p-type interlayer 21including the sublayers 60.

In an embodiment, a thickness of each of the plurality thin sublayers 60is at most 1 nanometer (nm). The plurality of thin sublayers 60 caninclude a single atomic plane, or include a few atomic planes. Forexample, the plurality of thin sublayers 60 can include five or feweratomic planes. In an embodiment, each of the plurality of thin sublayers60 can form a layer of discontinuous islands and/or a layer withopenings. A distance between each of the plurality of thin sublayers 60can be selected to be smaller than a critical distance for the creationof threading dislocations in the region between each of the plurality ofthin sublayers 60. The critical distance can be evaluated analyticallyfrom the energy balance calculations based on the lattice mismatchparameter for the corresponding semiconductor forming the electronblocking layer 20 and/or the p-type interlayer 21. The spacing betweenthe plurality of thin sublayers 60 does not have to be uniform and canbe varied. Regardless, in an embodiment, the spacing between eachadjacent pair of thin sublayers 60 is smaller than the critical distancefor the creation of threading dislocations. The plurality of thinsublayers 60 can help to manage the stresses within the electronblocking layer 20 and/or the p-type interlayer 21.

The plurality of thin sublayers 60 can comprise AlN layers, or layerswith a wide bandgap that contain a large molar fraction of aluminum. Inan embodiment, the sublayers 60 can comprise Al_(x)Ga_(1-x)N orAl_(x)In_(y)Bn_(z)Ga_(1-x-y-z)N, where the molar fractions x, y, and zare chosen to yield a band gap of the sublayer 60 that is higher than aband gap of the surrounding material (e.g., the electron blocking layer20 or the p-type interlayer 21). In an embodiment, the band gap of atleast one of the plurality of thin sublayers 60 is at least 1 eV higherthan the bandgap of the surrounding material (e.g., the material of theelectron blocking layer 20 or the material of the p-type interlayer 21).Alternatively, in an embodiment, the lattice constant of the pluralityof thin sublayers 60 is selected to be smaller than the lattice constantof the surrounding material. If the lattice constant of the plurality ofthin sublayers 60 is given by As, the lattice constant of surroundingmaterial is given by Ab and the lattice constant of AlN is Al, then inan embodiment, As<0.8*Ab+0.2*Al. In another embodiment, the plurality ofthin sublayers 60 can include an aluminum molar fraction that is atleast 10% higher than the aluminum molar fraction within electronblocking layer 20. It is understood that the plurality of thin sublayers60 can be selected to vary in composition and/or thickness throughoutthe entire structure. In an embodiment, the composition of the pluralityof thin sublayers 60 can be different from one sublayer to another. Inanother embodiment, the spacing between each of the plurality of thinsublayers 60 can change from one pair of adjacent sublayers to another.For example, the plurality of thin sublayers 60 in the electron blockinglayer 20 can be different than the plurality of thin sublayers 60 in thep-type interlayer 21. In an embodiment, the plurality of thin sublayers60 can be n-type doped with a dopant concentration between approximately10¹⁷ to approximately 10¹⁹ dopants per centimeter. The n-type dopant caninclude, for example, Si dopants, and/or the like.

In an embodiment, the plurality of thin sublayers 60 can include GaNrich sublayers that have a narrower band gap than the surroundingmaterial. For example, GaN rich sublayers can be introduced into alayer, such as the electron blocking layer 20, the p-type interlayer 21,and/or the p-type contact layer 22. In an embodiment, the GaN richsublayers can comprise Al_(x)Ga_(1-x)N orAl_(x)In_(y)Bn_(z)Ga_(1-x-y-z)N, where the molar fractions x, y, and zare chosen to yield a band gap that is narrower than the band gap of thesurrounding material (e.g., the material of layers 20, 21, or 22). In analternative embodiment, the lattice constant of the GaN rich sublayer isselected to be smaller than the lattice constant of the surroundingmaterial. If the lattice constant of a sublayer is given by As, thelattice constant of the surrounding material is given by Ab, and thelattice constant of GaN is Ag, then in an embodiment As>0.8*Ab+0.2*Ag.The GaN rich sublayers can also be introduced to control the stresseswithin the electron blocking layer 20, where the composition, width, andspacing of the GaN rich sublayers depends upon the device wavelength andthe thickness of the layers 20, 21, 22. The GaN sublayers can be heavilydoped with a p-type (Mg) dopant concentration of at least approximately5×10¹⁹ dopants per cubic centimeter. The GaN sublayers can be grown in atemperature range of approximately 950 degrees Celsius to approximately1050 degrees Celsius. Similar to the AlN layers, GaN sublayers caninclude a single atomic plane, or include a few atomic planes. Forexample, the GaN sublayers can include five or fewer atomic planes.Regardless, in each embodiment, the plurality of thin sublayers 60 arespaced throughout the electron blocking layer 20 (FIG. 12A), or theelectron blocking layer 20 and the p-type interlayer 21 (FIG. 12B), toreduce stress and to not create additional dislocations. In anembodiment, the plurality of thin sublayers 60 are spaced in a way as tonot cause formation of any additional dislocations due to latticemismatch between the plurality of thin sublayers 60 and the electronblocking layer 20.

FIGS. 13A and 13B show illustrative composition doping and temperatureprofiles for growth of a p-type contact layer 22 according toembodiments. During the epitaxial growth of the p-type layers 20, 21,22, the p-type doping and the temperature used for epitaxial growth canbe varied. In the embodiment shown in FIG. 13A, in addition toincreasing the p-type doping profile 42, the temperature is decreased asshown by the temperature profile curve 62. The temperature can beapproximately 1000 degrees Celsius to approximately 1150 degrees Celsiusfor epitaxial growth of the electron blocking layer 20, and decreaseduring growth of the p-type interlayer 21, and reach its lowest value ofapproximately 850 degrees Celsius to 1050 degrees Celsius after growthof the p-type interlayer 21 and the start of growth of the p-typecontact layer 22. It is understood that the temperature profile 62 shownin FIG. 13A only corresponds to a particular embodiment and that otherembodiments including different temperature profiles are possible.

In an embodiment, the p-type contact layer 22 is grown in a temperaturerange of approximately 950 degrees Celsius to approximately 1050 degreesCelsius. In another embodiment, the p-type contact layer 22 is grown ina temperature range of approximately 950 degrees Celsius toapproximately 1020 degrees Celsius. In yet another embodiment, if theelectron blocking layer 20 is grown at a temperature T1 range, thep-type interlayer 21 is grown at a temperature T2 range, and the p-typecontact layer 22 is grown at a temperature T3 range. In an embodiment,the high and low values of the T2 range is at least 1% lower than the T1and T3 range. For example, the temperature when growing the p-type layer21 can be lower than the temperature when growing the electron blockinglayer 20 and the p-type contact layer 22.

In another embodiment, the T1 and T2 ranges are non-overlapping rangesand the T2 and T3 ranges are non-overlapping ranges. In yet embodiment,when growing the p-type contact layer 22, the temperature can start atapproximately 950 degrees Celsius to approximately 1020 degrees Celsius.For example, the T1 range can be approximately 1000 degrees Celsius toapproximately 1150 degrees Celsius, the T2 range can be approximately850 degrees Celsius to approximately 1150 degrees Celsius, and the T3range can be approximately 850 degrees Celsius to approximately 1050degrees Celsius. At the end of the growth of the p-type layer 22, thetemperature can be approximately at least approximately 1050 degreesCelsius.

In an embodiment in which one or both layers 20, 21 include GaNsublayers (e.g., as shown in FIGS. 12A and 12B), the GaN sublayers canbe grown in a temperature range of T4, which is approximately 850degrees Celsius to approximately 1050 degrees Celsius. The reduction intemperature prevents the diffusion of dopants into the active region 18.The dopant concentration can be lowest when the temperature is high, andincrease for the lowest temperatures. In the embodiment shown in FIG.13A, the temperature profile 62 is a decreasing function through thep-type interlayer 21. In an embodiment, the temperature profile 62 mightexhibit a linear decrease with thickness of the p-type interlayer 21.However, it is understood that this is only illustrative, and thedecrease is not required to be linear.

In the embodiment shown in FIG. 13B, co-doping can be used to neutralizepoint defects. In this case, a second doping profile 64 is shown in anaddition to the p-type doping profile 42. The second doping profile 64can introduce an n-type dopant, such as Si, Ge, and/or the like, intothe electron blocking layer 20 and/or the p-type interlayer 21 followingelectron blocking layer 20 to further prevent the diffusion of thep-type dopant (e.g., Mg) into the active region 18. In general, then-type doping can have any monotonically decreasing characteristic,e.g., as shown by the doping profile 64 in FIG. 13B. More specifically,the n-type doping profile 64 can be sufficiently small in the p-typecontact layer 22 and considerably small in the p-type interlayer 21 toallow for appropriate p-type conductivity. In an embodiment, a magnitudeof the n-type doping is at most 20% of the magnitude of the p-typedoping. In a further embodiment, the magnitude of the n-type doping isone or two orders of magnitude smaller than the magnitude of the p-typedoping throughout the last half of the p-type interlayer 21 andthroughout the p-type contact layer 22. Diffusion of Mg into the activeregion 18 may actively utilize dislocation channels. The presence of anadditional dopant, such as Si, can deter Mg atoms from diffusing throughthese channels. The n-type doping 64 is larger in the electron blockinglayer 20 and decreases in the p-type contact layer 22. In an embodiment,when the electron blocking layer 20 and the p-type contact layer 22 bothinclude p-type and n-type doping, the n-type doping magnitude can be atmost approximately 20% of the p-type doping magnitude. Further in thisembodiment, the p-type doping shape profile can coincide with the n-typedoping shape profile apart from a phase factor wherein the phase factorcorresponds to the shift of one profile relative to another by a certainvertical distance. The shift can be between approximately 0.2 nanometersand approximately 10 nanometers.

In any of the embodiments discussed herein, a lateral variation ofp-type doping in any of the layers can be achieved through ionimplantation. In an embodiment, the ion implantation can be achievedthrough a photolithography process where some of the semiconductordomains are masked. For example, the p-type contact layer 22 cancomprise a mesa region with side walls. In this case, the lateraldistribution of the p-type doping can be selected to minimize currentcrowding in the proximity of the side walls of the mesa region. Inanother embodiment, variation in the lateral composition of a layer canbe achieved through an epitaxial lateral overgrowth (ELO) technique. Thelateral composition of at least one of the layers can vary along thelateral dimensions of the layer by at least one percent. In anembodiment, the layer with variable composition can be the electronblocking layer 20.

A heterostructure described herein can be fabricated using any solution.For example, the various semiconductor layers described herein can begrown over a substrate using an epitaxial growth procedure. As describedherein, one or more of the growth conditions utilized for a layer can beconfigured to create a desired property for the layer. For example, whena layer includes a superlattice, the growth conditions of such asuperlattice can affect the stresses and strains in the superlatticelayers. In particular, such a superlattice can comprise compressive andtensile layers that can further control the distribution of stressesthroughout the heterostructure. Furthermore, the tuning of stresses andstrains can be achieved by controlling the epitaxial growth parametersof a semiconductor layer. For example, by inducing three-dimensionalgrowth of a semiconductor layer, the resulting layer can exhibitcompositional and/or doping concentration inhomogeneities that canaffect the resulting stresses and strains within the layer. Furthermore,such inhomogeneities can result in regions that are highly transparentinterchanging with regions that are highly conductive, where the term“highly” is understood as being a relative term when comparing thetransparent or conductive properties of the different regions within thesemiconductor layer.

In one embodiment, the invention provides a method of designing and/orfabricating a circuit that includes one or more of the devices designedand fabricated as described herein. To this extent, FIG. 14 shows anillustrative flow diagram for fabricating a circuit 126 according to anembodiment. Initially, a user can utilize a device design system 110 togenerate a device design 112 for a semiconductor device as describedherein. The device design 112 can comprise program code, which can beused by a device fabrication system 114 to generate a set of physicaldevices 116 according to the features defined by the device design 112.Similarly, the device design 112 can be provided to a circuit designsystem 120 (e.g., as an available component for use in circuits), whicha user can utilize to generate a circuit design 122 (e.g., by connectingone or more inputs and outputs to various devices included in acircuit). The circuit design 122 can comprise program code that includesa device designed as described herein. In any event, the circuit design122 and/or one or more physical devices 116 can be provided to a circuitfabrication system 124, which can generate a physical circuit 126according to the circuit design 122. The physical circuit 126 caninclude one or more devices 116 designed as described herein.

In another embodiment, the invention provides a device design system 110for designing and/or a device fabrication system 114 for fabricating asemiconductor device 116 as described herein. In this case, the system110, 114 can comprise a general purpose computing device, which isprogrammed to implement a method of designing and/or fabricating thesemiconductor device 116 as described herein. Similarly, an embodimentof the invention provides a circuit design system 120 for designingand/or a circuit fabrication system 124 for fabricating a circuit 126that includes at least one device 116 designed and/or fabricated asdescribed herein. In this case, the system 120, 124 can comprise ageneral purpose computing device, which is programmed to implement amethod of designing and/or fabricating the circuit 126 including atleast one semiconductor device 116 as described herein.

In still another embodiment, the invention provides a computer programfixed in at least one computer-readable medium, which when executed,enables a computer system to implement a method of designing and/orfabricating a semiconductor device as described herein. For example, thecomputer program can enable the device design system 110 to generate thedevice design 112 as described herein. To this extent, thecomputer-readable medium includes program code, which implements some orall of a process described herein when executed by the computer system.It is understood that the term “computer-readable medium” comprises oneor more of any type of tangible medium of expression, now known or laterdeveloped, from which a stored copy of the program code can beperceived, reproduced, or otherwise communicated by a computing device.

In another embodiment, the invention provides a method of providing acopy of program code, which implements some or all of a processdescribed herein when executed by a computer system. In this case, acomputer system can process a copy of the program code to generate andtransmit, for reception at a second, distinct location, a set of datasignals that has one or more of its characteristics set and/or changedin such a manner as to encode a copy of the program code in the set ofdata signals. Similarly, an embodiment of the invention provides amethod of acquiring a copy of program code that implements some or allof a process described herein, which includes a computer systemreceiving the set of data signals described herein, and translating theset of data signals into a copy of the computer program fixed in atleast one computer-readable medium. In either case, the set of datasignals can be transmitted/received using any type of communicationslink.

In still another embodiment, the invention provides a method ofgenerating a device design system 110 for designing and/or a devicefabrication system 114 for fabricating a semiconductor device asdescribed herein. In this case, a computer system can be obtained (e.g.,created, maintained, made available, etc.) and one or more componentsfor performing a process described herein can be obtained (e.g.,created, purchased, used, modified, etc.) and deployed to the computersystem. To this extent, the deployment can comprise one or more of: (1)installing program code on a computing device; (2) adding one or morecomputing and/or I/O devices to the computer system; (3) incorporatingand/or modifying the computer system to enable it to perform a processdescribed herein; and/or the like.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims.

What is claimed is:
 1. A heterostructure comprising: an active region; ap-type contact layer having a p-type contact layer dopant concentration;an electron blocking layer located between the active region and thep-type contact layer; and a p-type interlayer located between theelectron blocking layer and the p-type contact layer, wherein the p-typeinterlayer is immediately adjacent to the electron blocking layer, andwherein the p-type interlayer includes a varying dopant concentrationthat increases from a dopant concentration comparable to a p-type dopantconcentration in the electron blocking layer to a dopant concentrationcomparable to the p-type contact layer dopant concentration in adirection from the electron blocking layer to the p-type contact layer,and wherein the p-type interlayer further includes a varyingsemiconductor alloy composition that decreases from a semiconductoralloy composition comparable to a semiconductor alloy composition of theelectron blocking layer to a semiconductor alloy composition comparableto a semiconductor alloy composition of the p-type contact layer in adirection from the electron blocking layer to the p-type contact layer.2. The heterostructure of claim 1, wherein the p-type dopantconcentration in the electron blocking layer is at most ten percent ofthe p-type contact layer dopant concentration.
 3. The heterostructure ofclaim 1, wherein an effective lattice constant for the p-type interlayeris between an effective lattice constant for the electron blocking layerand an effective lattice constant for the p-type contact layer.
 4. Theheterostructure of claim 1, wherein the varying dopant concentration inthe p-type interlayer increases once the varying semiconductor alloycomposition in the p-type interlayer is comparable to the semiconductoralloy composition of the p-type contact layer.
 5. The heterostructure ofclaim 1, further comprising a plurality of thin sublayers located withinthe electron blocking layer, each thin sublayer having an aluminum molarfraction that is at least 10% higher than an aluminum molar fraction ofa remainder of the electron blocking layer.
 6. The heterostructure ofclaim 5, further comprising a second plurality of thin sublayers locatedwithin the p-type interlayer.
 7. The heterostructure of claim 5, whereineach of the plurality of thin sublayers have a n-type dopantconcentration of approximately 10¹⁷ to approximately 10¹⁹ dopants percubic centimeters.
 8. The heterostructure of claim 5, a plurality ofgallium nitride (GaN) sublayers located within the electron blockinglayer.
 9. The heterostructure of claim 8, wherein each of the pluralityof GaN sublayers have a p-type dopant concentration of approximately5×10¹⁹ dopants per cubic centimeters.
 10. The heterostructure of claim1, wherein the electron blocking layer, the p-type interlayer, and thep-type contact layer each include a n-type dopant concentration.
 11. Theheterostructure of claim 10, wherein a magnitude of the n-type dopantconcentration is at most approximately 20% of a magnitude of the p-typedopant concentration.
 12. An optoelectronic device comprising: an n-typecontact layer having an n-type doping; a p-type contact layer having ap-type contact layer dopant concentration; an active region locatedbetween the n-type contact layer and the p-type contact layer; anelectron blocking layer located between the active region and the p-typecontact layer; and a p-type interlayer located between the electronblocking layer and the p-type contact layer, wherein the p-typeinterlayer is immediately adjacent to the electron blocking layer, andwherein the p-type interlayer includes a varying dopant concentrationthat increases from a dopant concentration comparable to a p-type dopantconcentration in the electron blocking layer to a dopant concentrationcomparable to the p-type contact layer dopant concentration in adirection from the electron blocking layer to the p-type contact layer,and wherein the p-type interlayer further includes a varyingsemiconductor alloy composition that continuously decreases from asemiconductor alloy composition comparable to a semiconductor alloycomposition of the electron blocking layer to a semiconductor alloycomposition comparable to a semiconductor alloy composition of thep-type contact layer in a direction from the electron blocking layer tothe p-type contact layer.
 13. The device of claim 12, wherein aneffective lattice constant for the p-type interlayer is between aneffective lattice constant for the electron blocking layer and aneffective lattice constant for the p-type contact layer.
 14. The deviceof claim 12, wherein the varying dopant concentration in the p-typeinterlayer increases once the varying semiconductor alloy composition inthe p-type interlayer is comparable to the semiconductor alloycomposition of the p-type contact layer.
 15. The device of claim 12,further comprising a plurality of thin sublayers located within theelectron blocking layer, each thin sublayer having an aluminum molarfraction that is at least 10% higher than an aluminum molar fraction ofa remainder of the electron blocking layer.
 16. The device of claim 15,wherein each of the plurality of thin sublayers have a n-type dopantconcentration of approximately 10¹⁷ to approximately 10¹⁹ dopants percubic centimeters.
 17. The device of claim 15, further comprising aplurality of gallium nitride (GaN) sublayers located within the electronblocking layer, and wherein each of the plurality of GaN sublayers havea p-type dopant concentration of approximately 5×10¹⁹ dopants per cubiccentimeters.
 18. The device of claim 12, wherein the electron blockinglayer, the p-type interlayer, and the p-type contact layer each includea n-type dopant concentration, and wherein a magnitude of the n-typedopant concentration is at most approximately 20% of a magnitude of thep-type dopant concentration.
 19. A method of fabricating a device, themethod comprising: creating a device design for the device using acomputer system, wherein the device design includes a heterostructurecomprising: an active region; a p-type contact layer having a targetp-type contact layer dopant concentration; an electron blocking layerlocated between the active region and the p-type contact layer; and ap-type interlayer located between the electron blocking layer and thep-type contact layer, wherein the p-type interlayer is immediatelyadjacent to the electron blocking layer, and wherein the p-typeinterlayer includes a varying dopant concentration that increases from adopant concentration comparable to a p-type dopant concentration in theelectron blocking layer to a dopant concentration comparable to thep-type contact layer dopant concentration in a direction from theelectron blocking layer to the p-type contact layer, and wherein thep-type interlayer further includes a varying semiconductor alloycomposition that continuously decreases from a semiconductor alloycomposition comparable to a semiconductor alloy composition of theelectron blocking layer to a semiconductor alloy composition comparableto a semiconductor alloy composition of the p-type contact layer in adirection from the electron blocking layer to the p-type contact layer;and providing the device design for use in fabricating the deviceaccording to the device design.
 20. The method of claim 19, wherein agrowth temperature for the electron blocking layer is higher than agrowth temperature for the p-type contact layer.